Check S/MIME signatures, and remember who signed

A signed message now says whether that holds up, as it is read. This is
verification only: nothing here signs, encrypts or decrypts, and the
private-key question that blocks those is untouched. Verifying needed
none of it, because the certificate travels inside the message -- which
is why this is the half that could be built.

What it checks. For multipart/signed carrying PKCS#7, the exact bytes of
the signed part -- headers included, canonicalised to CRLF -- are hashed
against the messageDigest attribute, and the signature over the signed
attributes is verified with WebCrypto against the certificate inside the
message. RSA PKCS#1 v1.5 and ECDSA over P-256/384/521, with SHA-256, 384
or 512.

The trust model is the design, and it is deliberately small. A browser
has no system trust store, and the certificate arrives inside the
message, so anyone can self-sign as anyone: on its own a good signature
shows only that the sender held the key they attached. So the word
"verified" is never rendered, and the reassuring case is not the loud
one. What carries the weight is remembering -- the first signed message
from an address pins its fingerprint, later ones are compared, and a
signer that changed is reported with both names and told to check by
another route. Trust on first use, no certificate authority anywhere.

The pins live in the account's settings rather than the browser: one
that only a single device knew would greet the same correspondent as new
everywhere else, which is how people are trained to click past the one
warning that matters. A pin records the message that created it, so the
message that established a signer keeps saying so instead of appearing
to be corroborated by itself -- without that, the very first signed
message anybody receives reads as "the same signer as before", where
before is itself. A changed, mismatched or expired signer is never
pinned, since writing the anomaly into the baseline makes every later
message agree with it.

Three things are declined rather than attempted, and all three say
"could not check" rather than "does not check out", because ignorance
and an accusation are different claims:

  - OpenPGP, by name. The signature carries no key and there is nowhere
    to get the sender's: x:PublicKey is the account's OWN registry, and
    a keyserver or WKD lookup would tell a third party who you
    correspond with -- the leak the image proxy exists to close.
  - SHA-1. Not forgeable in practice today, still not something to put a
    tick beside.
  - RSA-PSS, whose salt length lives in parameters this does not read.
    Guessing wrong would report a good signature as bad.

Nothing validates a chain: no CA bundle is shipped and revocation is not
checked. "Issued by" reports what the certificate claims, and a
self-signed one claims itself.

The DER, CMS, X.509 and MIME readers are hand-written and deliberately
narrow -- no new dependency, and the whole verifier is a lazily imported
8.6 kB chunk that a reader of unsigned mail never downloads. The one
place this is easy to get quietly wrong has its own function and its own
test: signed attributes are signed as a SET OF, not as the [0] IMPLICIT
they arrive as, and hashing the message instead would make every
signature "pass".

Tested against real `openssl smime -sign` output rather than hand-built
fixtures -- RSA, ECDSA, a tampered copy, and a valid signature by a
certificate for somebody else -- because a signed message written by
hand only agrees with whatever its author believed the format to be.
Also driven in a browser against the mock, which now serves three real
signed messages so every branch of the banner is reachable.

Translations: 34 new strings in all nine catalogues, 306 entries.
Falling back to English is unchanged at 24 per language.

KNOWN-ISSUES.md

@@ -48,6 +48,9 @@ works the same way — and dropped where 0.15 was the whole subject. Support for
   - **A create answers with the id alone**, no `createdAt`, so anything that reads the date back out of the create response gets `undefined`. **Patching `key` on an existing entry is allowed**, which is worth knowing and probably worth not doing: replacing a key by adding one and removing the old keeps `createdAt` meaning what it says.
   - **`expiresAt` is the registry's own field and is not derived from the key.** A certificate valid for a year registers with `expiresAt: null`. Reading the real date means parsing the certificate, and a date a client extracted would disagree with the server's field the moment the two ever differed.
 
+- **Signature checking is done here, and its trust model is deliberately small.** Stalwart does not verify S/MIME or OpenPGP signatures and exposes no result for one, so ihasmail does it in the browser: raw message, MIME split, PKCS#7 parse, WebCrypto. What is worth knowing is what it does *not* do, because the gap is a design choice rather than an omission. **No chain of trust is validated** — a browser has no system trust store, no CA bundle is shipped, and revocation is not checked — so a verified signature on its own shows only that the sender held the key inside their own message, which anyone can self-sign. What carries the weight instead is trust on first use: the first signed message from an address pins its fingerprint in the account's settings, and a later message signed by a different certificate is reported loudly. That is why the interface never says the bare word "verified", why a first sighting is grey rather than green, and why a changed signer never overwrites the pin. Verified against real `openssl smime -sign` output rather than hand-built fixtures — RSA and ECDSA, plus a tampered copy — because a signed message written by hand only ever agrees with whatever the author believed the format to be.
+- **OpenPGP signatures cannot be checked at all, for a reason that is not effort.** A PGP signature carries no key, so verifying one needs the sender's public key in advance, and there is nowhere to get it: `x:PublicKey` holds the *account's own* keys, not correspondents'. Fetching from a keyserver or via WKD would tell a third party who you correspond with each time you opened a message — the same leak the image proxy exists to close — so it is not done. Such a message says so by name rather than failing as an unknown format, and it says *could not check* rather than *did not check out*, which is a distinction worth keeping: one is ignorance and the other is an accusation.
+- **Two signature shapes are declined rather than attempted.** SHA-1 signatures are refused outright — one nobody can forge in practice today is still not one to put a tick beside. RSA-PSS is declined because the salt length lives in parameters ihasmail does not read, and guessing wrong would report a perfectly good signature as *bad*, which is a far worse thing to say than "cannot check". Both are shown as uncheckable, not as broken.
 - **Read receipts are built here, not by the server** — JMAP has an extension for them, [RFC 9007](https://www.rfc-editor.org/rfc/rfc9007.html)'s `MDN/send`, and Stalwart does not implement it: `urn:ietf:params:jmap:mdn` is not among its capabilities. So ihasmail assembles the `multipart/report` itself and sends it the long way round — raw MIME uploaded as a blob, `Email/import`, then `EmailSubmission` — which is also why the receipt lands in Sent, where it honestly belongs. Non-ASCII parts are base64 rather than `8bit`, so nothing depends on 8BITMIME surviving every hop. There is deliberately no "always send" setting: a receipt confirms to whoever asked that the address is live and when it was read, to an address of the sender's choosing, so each one is a decision. Verified against the mock end to end (upload, import, submit, `$mdnsent`), and **confirmed live on 0.16.19 (2026-08-26)**: a receipt asked for by a real sender was assembled, uploaded, imported and submitted, landed in Sent, and set `$mdnsent` so a second look does not offer to send another.
 - **Where 0.16 advertises `urn:stalwart:jmap`** — not where a JMAP client would look, and this now decides whether a sign-in is allowed at all. Stalwart builds the session-level `capabilities` from a fixed list (`Session::new`, plus WebSocket) that has never contained this capability, in any 0.16.x from 0.16.0 to 0.16.19. It hands it out per-account instead, so it appears in `primaryAccounts` and in each account's `accountCapabilities`. ihasmail tested for it in `capabilities` alone, which made every real 0.16 server read as older than 0.16 — and that one check drove three things: self-service credentials fell back to `POST /api/account/auth`, which 0.16 removed, so password changes, 2FA and app passwords all failed with "this mail server does not offer self-service credential management"; About reported the wrong generation; and Files took the older code path. It now looks in all three places, and is covered by tests on each. Worth restating plainly, because the stakes went up when 0.15 support was dropped: there is no longer a fallback path for this check to be wrong *into*. Getting it wrong now refuses every sign-in against a perfectly good server — a loud failure rather than a quiet misrouting, which is the trade the removal was making.
 - **HTML signatures** — Stalwart caps a signature at 2047 **bytes** (`value.len() < 2048` on a Rust string, so UTF-8 bytes, not characters). ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker; other clients see a text fallback. Confirmed live on 0.15.5 (2026-08-24): oversized, non-ASCII and inline-image signatures all save, and a test message arrived intact at Gmail with the logo inline.